Memory cells are formed when a B-lymphocyte successfully binds to an antigen (foreign molecule, often a surface protein of a pathogen) . The B-lymphocyte divides, producing plasma cells, which produce antibodies which bind to the antigen producing an immune response, and memory cells, which stay in the bloodstream for a long time. If the same antigen reappears the large number of memory cells in the bloodstream means that the antigen will ecounter a memory cell, and start an immune response faster than the original time when there were relatively few of that kind of B-lymphocyte. This is the basis of immunisation.